CEO tenure varies greatly depending on location, sector and management style, according to a study by Booz & Co.

The study found that external hires lasted only 4.3 years on average, while internal hires lasted 7.1 years.

Japanese companies saw a 18% CEO turnover rate in 2010, while American companies saw 12% turnover, and Chinese companies saw only 5% turnover.

The energy sector was the most brutal: CEO turnover rate was 16.3%, versus 11.6% in all sectors globally. CEOs in that sector also were more likely to have been forced out of their jobs.

The Booz study also found that CEOs who were more "hands on" were likely to get the boot sooner. Basically there are four types of CEO. Holding company CEOs have a minimal degree of involvement (think Berkshire Hathaway), strategic and active management companies involve progressively more guidance from the CEO (think LG Corp. and McKessen), whereas operationally involved companies rely on the CEOs for major guidance (think Ford).

Operationally involved CEOs are more likely to hold the chairman title, become involved in power struggles with the board, and their companies are more likely to get acquired, putting the CEO out of a job. More than one third of operational CEOs are replaced within four years. Of course, although there are greater risks with being more involved as a CEO, you also get more credit when things go well -- just look at Alan Mulally of Ford.

CEO tenures have shortened in the past decade. Executives now stay in the top post for an average of 6.6 years as opposed to 8.1 years.